In order to obtain a dense fluidized bed, traversed by a fluid flow, the pressure exerted by the fluid on solid particles must be compensated for by an opposite force, that is gravity in conventional fluidized beds, and centrifugal force in rotating fluidized beds. If the particles are very small, for example powders of group C of the Geldart classification, or if the density of the fluid is close to that of the solid particles, for example when the fluid is a liquid, the centrifugal force must be very high and/or the fluid flow passing through the fluidized bed must be low.
It is easy to obtain a rotating fluidized bed by supporting it on a rotating porous cylinder, but non-uniformity of the fluidized bed may bring about strong vibrations limiting the speed of rotation of this device and it is not easy to feed in and discharge solid particles without their being entrained by the fluid, when the particles are very fine. It is also possible to obtain a rotating fluidized bed in a fixed circular chamber by injecting fluid in thin layers along the fixed circular wall and by discharging it through openings in a central chimney passing through the circular chamber. Such a device is described in international patent application WO-A-2005/099887 in the name of one of the inventors of the present application.
In this latter device, in order to increase the speed of rotation of solid particles, the fluid flow must be increased and/or the distance between the circular wall and the central chimney must be reduced and/or the cross section of the fluid injectors must be reduced so as to increase the speed of injection at a constant flow rate. In practice, friction of the fluid along the central chimney makes it difficult to obtain a very high centrifugal force close to the central chimney with too high a fluid flow rate, which may bring about entrainment of the finer solid particles in the central chimney.